I think you may have got the John Mountains a little mixed
John Mountain who married Martha Mowbray in 1823 was born c.1800 so would not be the John Mountain who married in 1793
However John Mountain who married Martha is very possibly the son of John who married Ann.
But John who married Ann in 1823 was a widower on marriage
I'll post a few details
Marriage Parish records
Spitalfields Christchurch Stepney
John Mountain widower of this Parish and Ann Newbell ofthe same were married by licence on 26 May 1793 by Samuel Henshall
witnesses Joseph Newbell & Robert newbell
it seems they had a son
Baptism Spitalfields
John Mountain of John , Mariner, Quaker St, by Ann
born 30 April 1799
baptism 28 Jun 1799
or is this his baptism John Fountain 27 may 1799 Boston parents John & Ann?
so was this son the John Mountain who married Martha Mowbray in 1823 Fishtoft Boston Lincs
1841 census is so faint John's occupation or yes or no to p.o.b. cannot be read.
(1841 census rounds ages of adults 15+ years down to the nearest 5 years and just states yes or no in answer to the question "were you born in the county you are living in?" which in 1841 was Lincolnshire
1841
Boston Lincs
John Mountain 40
Martha 39
Frederick 13
Ann 12
Martha 8
Robert 6
Samuel 5
Joseph 3
James 2
HO107 613 5 1
(no relationships are given either on this census)
Sadly John died before the 1851 census - possibly the death reg Sept qtr 1838 Boston reg district is his
1851
Market Place Boston Lincolnshire
Martha Mountain Head 49 widow
Frederick son U 23 mariner (baptised 24 Aug 1827 as Frederick Newbell Mountain -so taking his grandmother's maiden name as a middle name)
Ann daughter 19 bapt 21 April 1831
Martha daughter 18 bapt 5 Feb 1833
Robert son 16 assistant brewer bapt 31 Oct 1834
Samuel son 15 Grocer's apprentice bapt 19 July 1936
Joseph son 13 (no bapt found)
James son 11 bapt 26 Aug 1839
William son 10 17 mar 1841
Rebecca daughter 8 bapt 28 mar 1843
Fanny daughter 5birth reg Dec qtr 1845
Mary Short servant 18
Alice Burton servant
all b Boston
baptised at St Botolphs Boston
also found these baptisms
Mowbray Mountain 13 Apr 1824
Susannah Mountain 30 Jun 1829
John Physick Mountain 20 Nov 1825
all to parents John & Martha
Hope this helps - also need to find John Mountain who married Ann Newbell 's first marriage
Suz